This invention relates to a cement accelerating agent to be added to cement of a Portland cement type for improving initial adhesion and initial setting of cement mortar or concrete for spraying on tunnel surfaces, side-slopes, structures and bridge understructures.
The cement referred to in the present invention is normal Portland cement and high early-strength Portland cement. Cement, other than used in Example 14, is "normal Portland cement."
In general, rapid development of initial setting is required for cement mortar and concrete. For example, when mortar or concrete is sprayed on a vertical or sloped surface of a tunnel ceiling or a bridge understructure, the mortar or concrete must develop a bonding strength to hold the mortar or the concrete itself on the ceiling surface within 30 seconds after spraying, and thereafter curing proceeds gradually to develop enough strength to hold the weight of the mortar or concrete.
Further, when mortar or concrete is sprayed on a tunnel ceiling where water flows out, on a loosened hill surface, for stopping water or for emergency repair, the start of initial setting in 1.5 to 3 minutes is too slow and unacceptable, and setting must start in seconds.
Heretofore, various cement accelerating agents have been added to mortar and concrete. Cement accelerating agents that have been used for the above purpose include (1) water glass-based agents, (2) aluminate-based agents, and (3) calcined alunite-based agents. However, the above cement accelerating agents have been found defective for the following reasons.
(1) Water glass-based agents
The strength does not increase after setting, there is insufficient long-term stability, there is a high pH of itself, and there is the possibility of Na pollution.
(2) Aluminate-based agents
The setting time and initial strength of the mortar or concrete to which a cement accelerating agent comprising sodium aluminate and sodium carbonate is added is greatly affected by the quantity of the added agent. This type of cement accelerating agent absorbs moisture and has a high pH value and provides insufficient storage stability. For example, an accelerating agent comprising calcium aluminate which is contained in aluminate cement, sodium sulfate and slaked lime has an insufficient initial setting property and insufficient storage stability. An accelerating agent comprising alkali aluminate and alkali carbonate causes abrupt variation in setting time and initial strength by the quantity of the added agent and has insufficient short-term strength. An accelerating agent comprising calcium haloaluminate and alkali carbonate rapidly sets after 30 seconds from spraying and tends to result in high scattering rate in spraying.
(3) Calcined alunite-based agents
An accelerating agent based on calcined alunite and gypsum has low initial adhesion and a slow initial setting property.
Thus, conventional cement accelerating agents have been found defective for the above-described reasons, and an accelerating agent that enhances the initial setting performance of mortar and concrete and has good durability is required.